dotnet/wpf · error · InvalidOperationException

InvalidOperationException(SR.Format(SR.XamlXmlWriterWriteNot…

Error message

InvalidOperationException(SR.Format(SR.XamlXmlWriterWriteNotSupportedInCurrentState, "WriteStartMember")) [implicit member, non-empty frame]

What it means

When WriteStartMember is called with an implicit property (e.g. an implicit collection/item member), XamlXmlWriter plans to merge it into the enclosing element's content. That only works when the current namespace-scope frame is empty (no open member framing); if a non-empty frame is active it throws InvalidOperationException 'WriteStartMember not supported in current state' because the implicit-member abbreviation cannot be nested inside another open member.

Solutions

Close the currently open member with WriteEndMember before starting the implicit member
Emit the property explicitly (real member) instead of relying on implicit-member merging when nesting is required
Re-derive the node stream from a XamlObjectReader so implicit members appear at valid positions

Example fix

// before
writer.WriteStartMember(items); // previous member still open
// after
writer.WriteEndMember();
writer.WriteStartMember(items); // implicit member now valid

Defensive patterns

Strategy: try-catch

Validate before calling

if (IsImplicit(property) && !currentFrameIsEmpty) throw new InvalidOperationException("Implicit member requires empty frame");

Type guard

bool CanStartImplicitMember(bool implicitProp, bool frameEmpty) => !implicitProp || frameEmpty;

Try / catch

try { writer.WriteStartMember(property); }
catch (InvalidOperationException ex) when (ex.Message.Contains("WriteStartMember")) { writer.WriteEndMember(); writer.WriteStartMember(property); }

Prevention

Close open members before starting implicit members
Prefer explicit members when nesting is required
Generate streams via XamlObjectReader

When it happens

Trigger: Calling WriteStartMember for an implicit property while an earlier member is still open in the current frame (namespaceScopes.Peek() not empty).

Common situations: Emitting nested implicit collections without closing prior members; hand-written node streams that skip WriteEndMember before starting an implicit member; markup expansion code writing members inside already-open members.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/1a0a3e57ad914536. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/XamlXmlWriter.cs:1041

                        Type = writer.namespaceScopes.Peek().Type,
                    });
                }

                writer.namespaceScopes.Peek().Member = property;

                XamlType parentType = writer.namespaceScopes.Peek().Type;
                if ((property == XamlLanguage.Items && parentType is not null && parentType.IsWhitespaceSignificantCollection) ||
                    (property == XamlLanguage.UnknownContent))
                {
                    writer.isFirstElementOfWhitespaceSignificantCollection = true;
                }

                XamlType containingType = writer.namespaceScopes.Peek().Type;
                if (IsImplicit(property))
                {
                    if (!writer.namespaceScopes.Peek().IsEmpty())
                    {
                        throw new InvalidOperationException(SR.Format(SR.XamlXmlWriterWriteNotSupportedInCurrentState, "WriteStartMember"));
                    }
                }
                else if (property == containingType.ContentProperty)
                {
                    if (!writer.namespaceScopes.Peek().IsEmpty())
                    {
                        throw new InvalidOperationException(SR.Format(SR.XamlXmlWriterWriteNotSupportedInCurrentState, "WriteStartMember"));
                    }
                    else
                    {
                        writer.currentState = TryContentProperty.State;
                        return;
                    }
                }
                else
                {
                    WriteMemberAsElement(writer);
                    writer.WriteDeferredNamespaces(XamlNodeType.StartMember);

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